Thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. An electronic picture can be subjected to color separation by color filters. The respective color-separated images can be converted into electrical signals. These signals can be operated on to produce electrical signals corresponding to various colors, for example, black, cyan, magenta, or yellow. These signals can be transmitted to a thermal printer. To obtain a print, a colored dye-donor layer, for example, black, cyan, magenta, or yellow, of a dye-donor element can be placed face-to-face with a dye image-receiving layer of a receiver element to form a print assembly, which can be inserted between a thermal print head and a platen roller. A thermal print head can be used to apply heat from the back of the dye-donor element. The thermal print head can be heated up sequentially in response to the various color signals. The process can be repeated as needed to print all colors, and a laminate or protective layer, as desired. A color hard copy corresponding to the original picture can be obtained. Further details of this process and an apparatus for carrying it out are described in U.S. Pat. No. 4,621,271 to Brownstein.
Thermal transfer works by transmitting heat through the dye-donor element from the back-side to the dye-donor layer. When the dyes in the dye-donor layer are heated sufficiently, they sublime or diffuse, transferring to the adjacent dye-receiving layer of the receiver element. The density of the dye forming the image on the receiver can be affected by the amount of dye transferred, which in turn is affected by the amount of dye in the dye-donor layer, the heat the dye-donor layer attains, and the length of time for which the heat is maintained at any given spot on the dye-donor element.
Current dye donor elements generally have numerous layers coated on each side of the support to enable good quality prints with desired characteristics and performance. It is advantageous to reduce the number of layers for both cost and performance because each layer in a dye-donor element can act as an insulator, slowing down the heat transfer through the layers of the dye-donor element to the receiver. At high printing speeds, and the resulting short application time, any reduction in heat transfer efficiency results in a lower effective temperature in the dye-donor layer during printing, which will result in a lower transferred dye density. One way to overcome the low print density associated with shorter line times is to reduce the thickness of the dye-donor layer, thus making the heat transfer efficiency greater.
Various other methods can also be employed to increase the heat transfer efficiency of the dye-donor element. Two such methods are increasing the print head voltage or increasing the relative amount of dye load. Both of these methods result in increased cost and other performance problems such as unwanted dye transfer and reduced print head lifetimes.
Thermal donors can be manufactured using various methods Typically, a substrate is formed by casting, and the dye layer is coated on the substrate by any known means, for example, gravure coating, spray coating, blade coating, or printing, such as ink jet printing. Substrates used for thermal donors are typically not extruded.
Various extruded films are known for other uses. For example, U.S. Pat. No. 6,599,383 B1 discloses a film comprising a first extruded layer including silicone oil, and a second extruded layer, wherein the first extruded layer has a lower coefficient of friction than the second extruded layer. Both layers include particles, preferably silica particles. U.S. Pat. No. 5,069,962 describes a biaxially oriented laminated film for magnetic recording material including a first layer containing a first thermoplastic resin as a major constituent, and a second layer containing a second thermoplastic resin comprising a crystalline polyester with inert particles. U.S. Pat. No. 5,236,768 discloses a heat resistant lubricating layer containing a modified silicone oil with a viscosity not lower than 600 cSt for use on the opposite side of the substrate from the dye layer of a thermal donor.
Typically slipping agents that have been used in extrusion for non-thermal imaging applications are fluoropolymer powders, for example, micronized polytetrafluoroethylene (PTFE), graphite, fatty acid esters, pentaerythritol, montanic acid esters, and sodium montanate. Silicones in the form of ultrahigh molecular weight siloxanes have also been used as lubricants.
A continuing problem of extruded films in the prior art is that they are insufficient to be used in thermal transfer because they stick to the print head during the printing process. It is desirable to have a film that has the dimensional stability of current base film supports while also incorporating slip agents to facilitate transport through the printer without annealing the film to the print head, thereby preventing stoppage of the process due to transport failure.